At the beginning of the year, Stacy Johnson wrote a piece for Money Talks News entitled "Things Babies Born in 2011 Will Never Know." Among the items he listed on the endangered list were videotape, travel agents, books, magazines, newspapers, movie rental stores, watches, paper maps, wired phones, long distance, newspaper classifieds (my 14-year-old daughter read this and asked me what one was), dial-up Internet, encyclopedias, CDs, film camera, catalogs, fax machines and wires. Should theatrical dimmers be on that list?
As quickly as LEDs have advanced in the last few years, it's not out of the realm of possibility that incandescent lamps and conventional dimmers could be overtaken by LEDs and pulse-width modulated dimming. But until that happens, there is more to know about dimmers and how to use them.
Dimming Pinspots
Although we've been using dimmers for years, I still get questions about whether or not it's okay to put a pinspot on a dimmer. Some people say it's not a good idea and others say they've been doing it for years and they're not going to stop doing it. What's the truth about dimming pinspots?
A pinspot is a small PAR can with a low-voltage, low wattage lamp. They have a very narrow beam that produces a pencil-thin shaft of light, or a pin beam – thus the name. They typically draw their power from the mains supply, so in order to change the voltage to the rated voltage of the lamp requires a transformer. If you were to open a pinspot by taking off the yoke and removing the front bezel and the lamp, you would find a small low-voltage transformer in it. This is the source of confusion about the nature of dimming a pinspot.
A conventional forward phase-control dimmer switches the input voltage from OFF to ON at very precise times. It happens once during the positive half cycle of the sine wave and once during the negative half cycle. By varying the firing angle of the switch, the dimming level can be set using a low-voltage control signal like DMX.
But when the voltage is switched on, it changes very rapidly from 0V to some positive or negative voltage level, depending on the firing angle. This very quick change in the voltage causes a sharp spike in the waveform and it can wreak havoc on the system with certain types of loads, including those with a transformer.
Reversing Directions
A transformer is an inductive load, and as such, it resists rapid changes in the current flowing through it. Think of it as a paddlewheel in a hydro system. Once a paddlewheel is set in motion by the flow of water (of in this case, electricity), if the water suddenly changes directions (as does alternating current or AC), then the paddlewheel has to first slow down before it can change directions. It can't instantaneously change directions without violating the laws of physics. The same is true of an electrical system. Once current is flowing through an inductor, the magnetic field that builds up around it resists a sudden change in direction or magnitude of the current.
In an electrical circuit, if the current through an inductor or transformer reverses direction or rises suddenly, then the magnetic field causes voltage to be fed back to the source. This is called "back EMF" or back electromagnetic force. (EMF is the same as voltage.) The back EMF can destroy circuit components like triacs and SCRs, which are the switches in forward phase-control dimmers.
The transformer can also be damaged by a dimming circuit if the waveform resulting from the switching is not exactly symmetrical with respect to zero volts. In other words, if the positive half is not the exact mirror image of the negative half, then it produces a DC "offset" current that flows through the transformer. A transformer is simply two back-to-back inductors or coils of wire, and a coil of wire presents virtually no resistance to DC. The only resistance it offers to DC is the resistance of the copper wire, which is tiny. Ohm's law (I = V ÷ R) says that for a certain amount of voltage, if the resistance is really, really small, then a huge amount of current will flow in the circuit. And that's what can happen with a dimmer controlling a transformer – a huge amount of current flows, and it burns up the transformer and/or the dimmer.
Testing the Theory
I did some tests with a pinspot on a Lex Slimmer Dimmer, and in the process, I found an interesting phenomenon. When the dimming level dropped to around 10 percent, the voltage waveform went completely haywire, and the transformer in the pinspot started buzzing loudly. Whatever the cause of the malfunction, the DC current in the circuit shot up, and if I had let it continue, it would have burned up the transformer. I suspect that's why dimmers designed for fluorescent lamps can only dim to 10 percent or, in some cases, 1 percent. They are probably built to avoid this from happening and damaging the circuit. The magnetic ballast in a fluorescent lamp is an inductor, just like the primary side of a transformer, and they are susceptible to the same dimming pitfalls as a pinspot.
Pinspots are great lighting tools. I love their pin-sized beams, and if you throw in a mirror ball, it can look really elegant in the right environment. If you have an array of pinspots, you can create some fabulous chases and effects. The icing on the cake is that pinspots are really inexpensive these days. I recently bought some for $10 apiece, and that was the retail price. If you're trying to design a lighting system on a budget, pinspots can really help you stretch the budget and fill out your design.
But don't let a $10 pinspot throw a spanner in the works and ruin a $200 dimmer module. You can use a dry contact closure to turn pinspots on and off, and if you must dim them, do it at your own peril, because it can cause damage to your equipment.
